1. Field of the Invention
The present invention relates to a semiconductor laser module.
2. Description of the Related Art
In the field of optical communications, an optical transmission system in accordance with wavelength division multiplexing (WDM) scheme has been developed with intention of increasing the capacity of transmitted information, and its growth is accelerating particularly in North America which takes the initiative.
As an excitation light source for an optical fiber amplifier used in the WDM scheme, there is an increasingly higher demand for pumping semiconductor lasers in 1480 and 980 nm band. Particularly, for wavelength multiplexing in the 1480 nm band and for flatness of the gain profile in the 980 nm band, the need is increased for stabilizing the wavelength of emitted light from a semiconductor laser module using a fiber bragg grating (hereinafter simply called the xe2x80x9cFBGxe2x80x9d).
The foregoing semiconductor laser module suffers from instability of a laser output in a semiconductor laser, for example, output light from the front or monitor light from the back, which causes a problem in controlling, for example, the output of an optical fiber amplifier. Particularly, the semiconductor laser module as described has a problem in that the stabilization of the wavelength using the FBG results in increasing the instability of the laser output.
It is an object of the present invention to provide a semiconductor laser module which is capable of stabilizing a laser output.
To achieve the above object, the present invention provides a semiconductor laser module comprising a semiconductor laser having a front end facet from which output light is emitted, an optical fiber, on which the output light emitted from the semiconductor laser is incident, having a light feedback portion for reflecting light at a predetermined wavelength, and a package for accommodating the semiconductor laser together with one end side of the optical fiber, wherein the light feedback portion includes a first light feedback portion set at a predetermined reflection center wavelength for determining an oscillating wavelength of the semiconductor laser, and at least one second light feedback portion.
Preferably, the second light feedback portion as used in the present invention has a reflectance equal to or lower than a maximum reflectance of the first light feedback portion at least within a wavelength range of a full width half maximum in the first light feedback portion.
Also preferably, the second light feedback portion has a reflection center wavelength substantially identical to that of the first light feedback portion.
Further preferably, a difference in the reflection center wavelength between the first light feedback portion and the second light feedback portion is set equal to or less than 0.5 nm.
Preferably, the first light feedback portion differs from the second light feedback portion in the reflection center wavelength.
Also preferably, the second light feedback portion has a flat reflection level characteristic substantially without wavelength dependence, and a reflectance lower than a maximum reflectance of the first light feedback portion.
Further preferably, either the first or the second light feedback portion comprises a fiber bragg grating.
Preferably, the first light feedback portion and an adjacent one of the second light feedback portions are spaced by a distance set equal to or longer than 5 mm between longitudinal center positions thereof.
Also preferably, two or more of light feedback portions chosen from the first light feedback portion and the at least one second light feedback portion comprise a chirped bragg grating having a reflection characteristic which is superposition of respective reflection characteristics.
Further preferably, the light feedback portion is positioned external to the package.
Preferably, the optical fiber includes polarization fluctuation preventing means disposed between the semiconductor laser and the light feedback portion for preventing fluctuations in light polarization.
Also preferably, the polarization fluctuation preventing means is a birefringent fiber.
Further preferably, the birefringent fiber is any of a PANDA fiber which has a core applied with a non-axially symmetric stress and a stress applying portion of circular cross-section; a bow-tie fiber having a fan-shaped cross-section; an elliptic jacket fiber of elliptic cross-section; and an elliptic core fiber having a core waveguide structure in a non-axially symmetric form and a core of elliptic cross-section.
Preferably, the distance from a rear end facet of the semiconductor laser to a longitudinal center position of either the first or the second light feedback portion is 10 cm or longer.
Also preferably, the semiconductor laser has an oscillating wavelength in a band centered at 980 nm.
Further preferably, the optical fiber is a lensed fiber, wherein light emitted from the semiconductor laser is coupled to a distal end of the lensed fiber.
Preferably, the lensed fiber is made by connecting by fusion the optical fiber formed with the first light feedback portion and/or the second light feedback portion.
According to these preferred aspects, it is possible to provide a semiconductor laser module which is capable of stabilizing a laser output.
The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description based on the accompanying drawings.